1. Field of the Invention
The present invention generally relates to a tape reproducing direction detecting apparatus and, more particularly, is directed to a tape reproducing direction detecting apparatus for a digital VTR (video tape recorder) of component system.
2. Description of the Prior Art
A D-1 type VTR (i.e., so-called 4:2:2 digital VTR) has been known to digitally record a component video signal (formed of a luminance signal Y and red and blue color difference signals R-Y and B-Y).
According to the D-1 type digital VTR, as shown in FIG. 1, a slant track called "program track" is comprised of two video sectors (half tracks) Sv for video data and four audio sectors Sa for audio data. An inclined angle (track angle) of this program track is a little larger than 5 degrees. A cue audio track Tka, a servo control track Tkc and a time code track Tkt are provided in the longitudinal direction of a tape T. The aforesaid track pattern is made common to both the 525/60 standard system (i.e., NTSC system) having 525 horizontal lines and a nominal field frequency of 60 Hz and the 625/50 standard system (i.e., PAL system) having 625 horizontal lines and a field frequency of 50 Hz.
The D-1 type digital VTR employs a 4-channel processing system in order to reduce a transmission rate of mass data. As, for example, shown in FIG. 2, four magnetic heads HA, HB and HC, HD are paired to provide two sets and two sets of magnetic heads HA, HB and HC, HD are mounted on a rotary drum D with an angular extent of 180 degrees. The magnetic tape T is wrapped around the rotary drum D over a tape wrapping angle of 180 degrees and two slant tracks shown in FIG. 1 are sequentially formed on the magnetic tape T.
Owing to the above-mentioned 4-channel processing, in the case of the 525/60 standard system, one field of video data is recorded over 20(=4.times.5) half tracks (video sectors) S1 to S20 corresponding to 10 slant tracks as shown by solid lines in FIG. 3. On the other hand, in the case of the 625/50 standard system, although the transmission rate of data is the same, the field period is longer so that one field of video data is recorded over 24 video sectors S1 to S24 corresponding to 12 slant tracks involving additional four video sectors S21 through S24 shown by phantoms in FIG. 3. In both cases, the hatched four sectors S1 to S4 correspond respectively to the four magnetic heads HA through HD and compose a set of segment. Also, the recording of one field of video data is started from the sectors S1 and S2 which are formed from substantially the center of the magnetic tape (upper side of FIG. 3) and ended with the ends of the sectors S19, S20 or S23, S24 which are formed near the control track Tkc (see FIG. 1).
The D-1 VTR carries out a pair of error corrections called "inner correction" and "outer correction" by utilizing a product block involving the Reed Solomon correction code. The outer code block is comprised of 30 bytes of video or audio data (column) and 2 bytes of Reed Solomon correction code added thereto, while the inter code block is comprised of 60 bytes of video or audio data (row) and 4 bytes of Reed Solomon correction code.
Video data are arrayed in each video sector in the units of a synchronizing (i.e., sync.) block shown in FIG. 4. Each sync. block is composed of 2 bytes of a sync. word, 4 bytes of a block identifier (ID) and two inner code blocks. Each identifier involves a sync. block ID for numbering respective blocks along a program track, a sector ID, a segment ID and a field ID for numbering respective sync. blocks in accordance with the sector, segment and field positions.
The field IDs shown by reference symbols F in FIG. 3 are repeatedly given at a 4-field cycle in the sequence of 0, 1, 2, 3 in that order. Also, the segment ID is given in the sequence of 0 to 4 in the case of the 525/60 standard system and is given in the sequence of 0 to 5 in the case of the 625/50 standard system.
Because of the aforesaid track pattern, the D-1 VTR cannot reproduce one field of video data by one revolution of the rotary drum D even in the normal playback mode. For this reason, a video frame memory having a capacity of 3 fields or more is mounted on the playback side, whereby one field of complete video data is stored in this video frame memory and then sequentially output. The 3 fields of video data are composed of 2 fields of write data and one field of read data and in the slow motion playback mode, a memory having a capacity of 2 fields is needed for write data.
In the slow motion playback mode and in the 1/n slow motion mode, it takes n fields of time to reproduce one field of video data. Then, during the present field of video data is reproduced, a preceding field of video data is repeatedly read out from the video frame memory.
When the dynamic tracking is not carried out, then each magnetic head obliquely scans the program tracks to reproduce data. Also in this case, by the aforementioned error correction, a perfect or substantially perfect reproduced picture can be obtained within the .+-.1/2 normal tape speed reproduction mode.
According to the conventional D-1 VTR, when the tape transport direction is changed in the forward and reverse directions particularly in the slow motion playback mode, then the reproducing direction is determined by the forward and reverse sequences of the field ID in the reproduced data to thereby control the write and read of the frame memory.
However, in the video tape edited in the unit of frame or field because of the purpose for producing a video program, and the sequences of the field IDs become discontinuous as in 0-1-0-1 before and after the editing point.
When such video tape is reproduced, then the conventional D-1 VTR errorneously determines that the tape running direction is changed in the forward, reverse and forward directions so that data are read out from the video frame memory in the sequence of, for example, the preceding field, the field preceding the preceding field and the preceding field, in that order. There is then the problem such that the motion of the reproduced picture is disturbed.
Further, according to the conventional D-1 VTR, the reproducing direction is determined by detecting the sequence of the field ID in the reproduced data so that the reproducing direction cannot be determined until the switching of the field ID is detected. Therefore, it is frequently observed that the switching control of the frame memory is delayed.
As a consequence, if the tape running direction is switched in the middle of one field period in the slow motion playback mode, then incomplete video data of less than one field is stored in the video frame memory and this incomplete video data is read out from the video frame memory, thereby a reproduced picture being disturbed.